1. Field of the Invention
The present invention relates generally to a BWA (Broadband Wireless Access) communication system using OFDM (Orthogonal Frequency Division Multiplexing), and in particular, to an apparatus and method for generating a preamble sequence using an ARM (Aperiodic Recursive Multiplex) code.
2. Description of the Related Art
In general, a wireless communication system supports wireless communications and includes Nodes B and UEs (User Equipments). A Node B and a UE use transmission frames to support the wireless communications. For transmission and reception of transmission frames, the Node B and the UE must mutually acquire synchronization. For this purpose, the Node B transmits a sync signal to inform the UE of the start of a transmitted frame. The UE then acquires frame timing from the sync signal and demodulates the frame according to the frame timing. A particular preamble sequence that is preset between the Node B and the UE is used as the sync signal. When the preamble sequence is transmitted as a burst signal, its reception performance depends on aperiodic autocorrelation characteristics.
An OFDM communication system uses a preamble sequence with a low PAPR (Peak to Average Power Ratio). The Node B transmits to the UE a short preamble sequence for coarse synchronization concatenated with a long preamble sequence for fine frequency synchronization. On the uplink, the UE transmits to the Node B a long preamble sequence for fine frequency synchronization. In the case of an STC (Space Time Code) frame directed from the Node B to the UE, an STC preamble sequence is configured with the same structure such that STC preamble-based synchronization acquisition and channel estimation is possible in UEs regardless of receiving STC signals or not receiving STC signals.
Similar to the preamble sequence for frame timing acquisition, there exists a sequence for burst synchronization acquisition. In a BWA communication system, for example, using FDD (Frequency Division Multiplexing), a plurality of burst slots exist in a frame. Therefore a slot sync signal is used to find the start of the slots, and a midamble signal exists in each burst slot for burst slot synchronization in the TDD communication system. The slot sync signal or midamble signal is also transmitted in the form of a preset sequence between the Node B and the UE, similar to the frame sync signal.
The OFDM BWA communication system transmits data in a time-multiplexed frame to a plurality of UEs. A frame preamble is transmitted for a predetermined time period from the start of the frame to identify the start of the frame and a burst preamble exists at the start of data due to intermittent data transmission for the UEs in the frame. To detect the start of the data transmission, the UEs receive data preambles. To synchronize to the start of data for data reception, the UEs acquire a preamble signal commonly used throughout the system.
The OFDM communication system employs the same source coding, channel coding, and modulation as non-OFDM communication systems. While data is spread prior to transmission in CDMA (Code Division Multiplexing), data is inverse-fast-Fourier-transformed and guard intervals are inserted into the data in OFDM. Accordingly, OFDM transmits broadband signals using relatively simple hardware. In the OFDM communication system, an RFFT (Inverse Fast Fourier Transformer) outputs a time domain signal for the input of parallel bit/symbol sequences after data modulation. The time domain signal contains multiplexed signals on sub-carriers divided from a broad band. Multiple modulations symbols for an OFDM duration are summed by the IFFT processing.
Transmission of IFFT OFDM symbols without any additional processing, however, causes interference between the previous OFDM symbol and the current OFDM symbol. To eliminate the intersymbol interference (ISI), guard intervals are inserted. The guard interval insertion is achieved by transmitting null data in corresponding positions, but wrong estimation of the start of an OFDM symbol in a receiver may cause interference between sub-carriers and increase an OFDM symbol decision error rate. Hence as a guard interval, a cyclic prefix or a cyclic postprefix is proposed. The cyclic prefix is to copy the last n/1 bits of a time-domain OFDM symbol and insert them in an effective OFDM symbol. The cyclic postprefix is to copy the first 1/n bits of a time-domain OFDM symbol and insert them in an effective OFDM symbol. The duplication and disposition of the first or last portion of an OFDM symbol in a particular OFDM symbol enables acquisition of OFDM symbol time/frequency synchronization in the receiver.
Meanwhile, a transmitted signal is distorted during transmission on a radio channel. The receiver acquires time/frequency synchronization and channel estimation using a preset preamble signal and demodulates the distorted data to symbols in a frequency domain by FFT (Fast Fourier Transform). Then the receiver decodes information data by subjecting the demodulated symbols to channel decoding and source decoding in correspondence with channel coding used in the transmitter.
The preamble signal is used for frame timing synchronization, frequency synchronization, and channel estimation in the OFDM communication system, although guard intervals and pilot sub-carriers can also be used for the same purpose. Known symbols are transmitted as a preamble signal at the start of each frame or data burst. Time/frequency/channel estimation information obtained from the preamble signal is then updated using a guard interval and a pilot sub-carrier in data of the frame.
The preamble sequence must be generated, taking into account the following parameters.
(1) The PAPR of OFDM symbols must be low to maximize the transmission efficiency of a power amplifier in the transmitter. OFDM symbols in the time domain at the output of an IFFT in the transmitter must have a uniform power distribution. In other words, if symbols with a low cross correlation in the frequency domain are input to the IFFT, the IFFT output has a low PAPR.
(2) Since the receiver estimates time/frequency information by cross correlation or autocorrelation, OFDM symbols in the time domain must be repeated. To do so, null data (zeros) are interpolated in a frequency-domain sequence.
(3) A virtual carrier or a guard band is defined to minimize adjacent channel interference. In OFDM being a multi-carrier system, null data is inserted in the outermost sub-carriers in the frequency domain to solve the problem of adjacent channel interference. The outermost sub-carriers are the middle indexes among IFFT input indexes.
Optimum communication performance is achieved only if the above three conditions are satisfied when a preamble sequence is generated in the OFDM communication system.
A frame structure with the above-described preamble will be described below with reference to FIG. 1. FIG. 1 illustrates the structure of a frame in a typical BWA communication system.
Referring to FIG. 1, reference numeral 101 denotes a frame preamble indicating the start of the frame to enable a UE to acquire frame timing synchronization to a Node B. The frame preamble 101 is modulated by BPSK (Binary Phase Shift Keying) or QPSK (Quadrature Phase Shift Keying). Reference numeral 102 denotes the frame containing real information data. The data in the frame 102 is modulated by QPSK, 16QAM (Quadrature Amplitude Modulation), or 64 QAM and then multiplexed in OFDM. N frames are illustrated in FIG. 1. This means that N channels transmit data following downlink/uplink preambles or STC preambles.
As described before, a preamble sequence preset between the transmitter (Node B) and the receiver (UE) is used for frequency synchronization and channel estimation. In view of the burst property of the preamble sequence, it requires a low PAPR and excellent performance in time/frequency synchronization and channel estimation.
FIG. 5 illustrates a preamble structure in the typical BWA communication system. Referring to FIG. 5, a short preamble 501 and long preambles 502 and 503 are used in the BWA communication system. The short preamble 501 is used for coarse synchronization between a Node B and a UE and the long preambles 502 and 503, for fine frequency synchronization. A downlink preamble being the short preamble 501 concatenated to a long preamble is at least 160 samples in duration, and an uplink burst preamble is at least 512 samples in duration. Reference numeral 504 denotes an STC preamble transmitted to UEs regardless of whether they support STC or not, for frequency synchronization acquisition and channel estimation. Hence a BWA system requires preamble signals of different lengths and thus requires an apparatus and method commonly used to generate the different preamble signals, particularly with excellent PAPR characteristics, and time/frequency synchronization and channel estimation performance.